1. Field of the Invention
The present invention relates to a paper feeder for an electrophotographic image forming apparatus in which a toner image is formed on a image carrier and rotates along with the image carrier, in particular to a paper feeder which supplies a sheet of paper to an image transfer position at which the toner image is transferred from the image carrier to the paper sheet.
2. Description of the Related Art
Conventionally, a laser printer contains a photosensitive drum which rotates in one direction at a time of printing. During the rotation, the photosensitive drum is sequentially subjected to processes of an electrifying charger, a laser scanner, a developing unit, a transferring charger, cleaning unit and a discharge unit, which are arranged in that order along the peripheral surface of the photosensitive drum. The surface of the photosensitive drum is electrified by the high voltage applied from the electrifying charger and is scanned by a laser beam irradiated from the laser scanner. The laser beam exposes the charged surface selectively, in accordance with print data for one page, and forms an electrostatic latent image on the photosensitive drum. The developing unit supplies toner to the drum surface and makes the electrostatic latent image visible as a toner image comprising toner adhered to the drum surface in accordance with the electrostatic latent image. The transferring charger charges a sheet of paper by applying high voltage to the paper supplied through a feeding path for the toner image, thereby transferring the toner image on the photosensitive drum to the paper by means of electrostatic suction force. The paper is ejected through a fixing unit which is provided for fixing the toner image onto the paper by means of heat and pressure. After transferring the toner image, the cleaning unit removes unused toner from the drum surface, and the discharge unit removes the electricity from the photosensitive drum surface.
Generally, the above mentioned laser printer uses a paper cassette, containing pre-cut sheets and attached to the printer. The papers are extracted one by one from the paper cassette by means of a pick up mechanism and are carried to the exhaust port of the printer through the transferring charger and the fixing unit by means of plural sets of feeding rollers. The feeding rollers begin to move at the same time as the photosensitive when each printing run starts, and the rotation of the rollers is maintained at a fixed speed throughout the processes. The pick up mechanism starts to move while the toner image is being formed, in response to the rotation of the photosensitive drum, and the timing of the start is fixed so that the paper is opposed to the toner image formed on the photosensitive drum when the toner image passes through the position of the transferring charger.
Incidentally, the above-mentioned processing sections and others are controlled by an electronic control circuit including CPU, the operation sequence of which is determined by a software program. The starting and stopping of the pick up mechanism in the conventional art is dependent on the time management of the CPU.
However, the conventional laser printer as mentioned above has a disadvantage in that the timing for supplying the paper to the image transfer position is not accurate. This is because of a variation of the overhead due to a modification of the software program, or a variation of the start timing of the pick up mechanism due to an interrupt handling which has no relationship to the control of the pick up mechanism. In such a case, it is not possible to transfer the toner image within the desired range of the paper. This problem becomes more serious in proportion to the speed of the printing operation.